The regional cerebral metabolic rate for glucose was measured with the [14C]2-deoxy-D-glucose technique in young and aged male Fischer-344 rates, following administration of the serotonin agonist m-chlorophenylpiperazine. Aged rats displayed reduced responsivity to m-chlorophenylpiperazine, indicating an age-dependent functional defect in serotonergic neurotransmission. The serotonin neurotoxin p-chloroamphetamine abolished metabolic effects of m-chlorophenylpeperazine, indicating that age related serotonin system of the rat brain. A model of cholinergic cortical deafferentation was implemented in rats, lesioning the nucleus basalis magnocellularis. Cerebral metabolic deficits were smaller in aged rats, demonstrating an age-related reduction in tonic cholinergic input to the neocortex.